1. Technical Field
Exemplary embodiments of the present invention relate to a wireless communication technology, and more particularly, to a multiple-input multiple-output (MIMO) system and a method of is generating a hierarchical codebook therefor.
2. Related Art
An example of methods for increasing data transmission efficiency in a wireless communication system may include a multiple-input multiple-output (MIMO) scheme. The MIMO scheme may be classified into a single-user MIMO (SU-MIMO) scheme and multiuser MIMO (MU-MIMO) scheme according to whether data transmission is enabled using the same resources when data are transmitted to each user.
Further, the MU-MIMO system may be classified into an open loop type in which a transmitting end performs communication without considering a channel state and a closed loop type in which a transmitting end performs communication by referring to channel information fedback from a receiving end. However, the closed loop type approaching theoretical transmission capacity by applying an independent modulation and coding scheme to each transmit antenna according to a channel state has been mainly used.
In the closed loop type MU-MIMO system, the channel information is used to improve performance of the entire system and channel state information (CSI) needs to be fedback from a receiving end to a transmitting end so as to acquire the channel information at the transmitting end.
Generally, a codebook is used to transmit the channel state information from the receiving end to the transmitting end. Each codeword configuring the codebook represents different channel is states for channels formed between the transmitting end and the receiving end. In particular, a frequency division duplex (FDD) system needs to accurately feedback the channel state information.
In the system using the limited feedback information, the receiving end estimates its own channel state information and searches a codeword approximating the channel information state estimated by a predefined distance measuring scheme. Further, an index of the searched codeword is fedback to the transmitting end.
An example of representative example of the codebook may include a Grassmannian codebook. The Grassmannian codebook has been known as being optimized for a independent identically distributed (i.i.d) channel, which leads to degraded performance in a spatially correlated channel.
Another representative example for generating the codebook may include a codebook based on discrete Fourier transform (DFT), which may be applied to both of the i.i.d. channel and the spatially correlated channel. However, the DFT based codebook increases calculation complexity and degrades efficiency as a size of the codebook is increased.
The DFT based codebook and the existing codebooks may reduce quantization errors as the size of the codebook is increased but may exponentially increase the calculation complexity for searching the codeword according to the size of the codebook. To this end, a scheme for hierarchically configuring the codebook has been researched and the DFT based codebook is also designed as the is hierarchical codebook. However, the hierarchical codebook proposed until now is for the multi-user MIMO system and therefore, can be hardly applied to the single-user MIMO system.